UV photosensitive melted glasses

ABSTRACT

The present invention relates generally to UV (ultraviolet) photosensitive bulk glass, and particularly to batch meltable alkali boro-alumino-silicate and germanosilicate glasses. The photosensitive bulk glass of the invention exhibits photosensitivity to UV wavelengths below 300 nm. 
     The photosensitivity of the alkali boro-alumino-silicate and germanosilicate bulk glasses to UV wavelengths below 300 nm provide for the making of refractive index patterns in the glass. With a radiation source below 300 nm, such a laser, refractive index patterns are formed in the glass. The inventive photosensitive optical refractive index pattern forming bulk glass allows for the formation of patterns in glass and devices which utilize such patterned glass.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/099,088, filed Mar. 15, 2002, to Borrelli etal., and entitled “UV Photosensitive Melted Glasses,” which is acontinuation-in-part of U.S. patent application Ser. No. 09/874,342,filed Jun. 5, 2001, now U.S. Pat. No. 6,632,759 to Borrelli, et al, andentitled “UV Photosensitive Melted Germano-Silicate Glasses,” whichclaims the priority of U.S. Provisional Patent Application Ser. No.60/221,811, filed Jul. 31, 2000. The inventions described in theseapplications are assigned to the assignee of the present invention, andthe disclosure of these applications are incorporated by referencesherein and for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to photosensitive bulk glass,and particularly to meltable alkali boro-alumino-silcate andgermanosilicate glasses.

BACKGROUND OF THE INVENTION

Optical transmission systems, including optical fiber communicationsystems, have become an attractive alternative for carrying voice anddata at high speeds. While the performance of optical communicationsystems continues to improve, there is increasing pressure on eachsegment of the optical communication industry to reduce costs associatedwith building and maintaining an optical network.

Quite often, optical communication systems require various types ofoptical filter elements. For example, diffractive filter elements may beused to effect the separation (demultiplexing) of individual wavelengthchannels in a wavelength division multiplexed (WDM) optical system. Inaddition, these refractive filter elements may be used to compensate forthe ill-effects of dispersion, to include chromatic dispersion (CD) andpolarization mode dispersion (PMD).

One type of diffractive optical filter is a Bragg grating. Bragggratings are interferometric optical devices which have been employed ina variety of applications including multiplexing/demultiplexingapplications and dispersion compensation applications. Bragg gratingsmay be used to reflect light of a wavelength which satisfies the Braggphase matching condition, and which transmits all other wavelengths.

One useful technique for forming a Bragg grating is to selectively alterthe index of refraction in a substrate in a periodic manner. Thisselective alteration of the index of refraction can be used to fabricateboth Bragg gratings in which the period of the index of refraction isregular, as well as chirped gratings in which the period of the index ofrefraction varies as a function of distance.

What is needed is a batch meltable glass material which isphotosensitive, and which overcomes certain drawbacks of conventionalglass materials.

SUMMARY OF THE INVENTION

The invention includes a photosensitive glass. According to an exemplaryembodiment of the present invention a meltable photosensitive glass hasa hydrogen content of greater than 10¹⁷ hydrogen molecules/cm³. Therefractive index change in the exposed portions of the glass is 10⁻⁴ (Δn>10⁻⁴) measured at a wavelength of 633 nm; and the glass isphotosensitive to light having a wavelength of less than 300 nm.

According to another exemplary embodiment of the present invention, thestarting glass is a photosensitizable alkali boro-alumino-silicate glassthat can be loaded with hydrogen to make it photosensitive. In oneexemplary embodiment of the present invention, the glass is a below 300nm photosensitive glass which has a composition of 40-80 mole % SiO₂,2-15 mole % GeO₂, 10-36 mole % B₂O₃, 1-6 mole % Al₂O₃ and 2-10 mole %R₂O where R is chosen from the alkali elements with the glass exhibitingphotosensitivity to below 300 nm wavelengths. In another exemplaryembodiment of the present invention, the glass has a compositionincluding approximately 25 weight % to approximately 45 weight % SiO₂,approximately 3 weight % to approximately 22 weight % GeO₂,approximately 7 weight % to approximately 28 weight % B₂O₃,approximately 6 weight % to approximately 22 weight % Al₂O₃,approximately 6 weight % to approximately 25 weight % R₂O wherein R isan alkali, and approximately 3 weight % to approximately 11 weight % F,with the glass exhibiting photosensitivity to below 300 nm wavelengths.

Another exemplary embodiment of the present invention includes amolecular hydrogen loadable photosensitive bulk glass. Thephotosensitive bulk glass is an alkali boro-alumino silicate glass witha melting temperature no greater than 1650° C. Preferably, the glass hasa batch composition comprising no greater than 85 mole % SiO₂, no lessthan 10 mole % B₂O₃, no less than 2 mole % GeO₂, and a combined alkaliand alumina content no greater than 20 mole % Al₂O₃+alkali with theglass having a molecular hydrogen loadable level of at least 10¹⁸H₂molecules/cm³.

Another exemplary embodiment of the present invention includes ameltable photosensitive germanosilicate glass material having a hydrogencontent less than 10¹⁷H₂ molecules/cm³. In one exemplary embodiment ofthe invention, the glass is a below 300 nm photosensitive glass whichhas a composition of 40-80 mole % SiO₂, 2-15 mole % GeO₂, 10-36 mole %B₂O₃, 1-6 mole % Al₂O₃ and 2-10 mole % R₂O where R is chosen from thealkali elements with the glass exhibiting photosensitivity to below 300nm wavelengths. In another exemplary embodiment of the invention, theglass includes approximately 25 weight % to approximately 45 weight %SiO₂, approximately 3 weight % to approximately 22 weight % GeO₂,approximately 7 weight % to approximately 28 weight % B₂O₃,approximately 6 weight % to approximately 22 weight % Al₂O₃,approximately 6 weight % to approximately 25 weight % R₂O wherein R isan alkali, and approximately 3 weight % to approximately 11 weight % F,with the glass exhibiting photosensitivity to below 300 nm wavelengths.

Another exemplary embodiment of the present invention includes a methodof making a refractive index pattern. The invention includes providing aphotosensitive bulk glass having a 300 nm absorption less than 20 dB/cm,providing a radiation source below 300 nm, forming a pattern with thebelow 300 nm radiation, and exposing the photosensitive bulk glass tothe pattern to form a modulated refractive index pattern in the bulkglass.

Another exemplary embodiment of the present invention includes a methodof making a molecular hydrogen loadable photosensitive glass opticaldevice preform. Preferably, the method comprises making a refractiveindex pattern preform out of melted glass. The method includes providinga germania silica glass powder batch with transition metal contaminationlevel ≦1 ppm by heavy metals. The method includes melting contaminationlevel ≦1 ppm by weight for heavy metals. The method includes melting thesilica glass powder batch to form a homogeneous glass melt, cooling theglass melt into a UV transmitting bulk glass having a 300 nm absorptionless than 20 dB/cm and forming the bulk glass into an optical devicepreform in which refractive index patterns can be made.

Another exemplary embodiment of the present invention includes aphotosensitive glass optical refractive index pattern preform for usewith UV light in the formation of refractive index patterns. The preformis comprised for use with UV light in the formation of refractive indexpatterns. The preform is comprised of an alkali boro-alumino-silicateglass with a 300 nm absorption less than 20 dB/cm. The preform glass hasa UV wavelength inducable modulated refractive index Δn level greaterthan 10⁻⁵ with a molecular hydrogen level of at least 10¹⁸H₂molecules/cm³.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read with the accompanying drawing figures. It is emphasized thatthe various features are not necessarily drawn to scale. In fact, thedimensions may be arbitrarily increased or decreased for clarity ofdiscussion.

FIG. 1 is a plot of absorbance/nm versus UV wavelength (nm) (200-300 nm)in accordance with the invention.

FIG. 2a is a plot of induced modulated refractive index [Δn(×10⁻⁴)]versus UV exposure time (minutes) in accordance with the invention.

FIG. 2b is a plot of induced modulated refractive index [Δn(×10⁻⁴)]versus UV exposure fluence [mJ/cm²] in accordance with the invention.

FIG. 3 is a photosensitivity thermal stability plot diffractionefficiency of induced refractive index changes in the bulk glass versushours heated at 400° C. in accordance with the invention.

FIG. 4 is a plot induced refractive index [Δn] versus OH concentrationin accordance with the invention. FIG. 4 inset is a plot of absorbanceversus wave numbers (cm⁻¹) showing OH stretching vibrations andabsorbance before (dashed line) and after (solid line) a 90 minute UVexposure of 20 mJ/cm²/pulse.

FIG. 5 is a plot of absorbance versus UV wavelength (nm) before (dashedline) and after (solid line) the 90 minute UV exposure of 20mJ/cm²/pulse of FIG. 4.

FIG. 6 is a plot of intensity (dBm) versus wavelength (1545nm-1559 nm)of refractive index pattern grating formed in the bulk glass inaccordance with the invention. FIG. 6 inset show the geometry of the UVexposure and the reflectivity and transmission measurements of the plot.

FIG. 7 illustrates the refractive index pattern grating of FIG. 6. FIG.7a is a cross-section showing the refractive index pattern grating inthe bulk glass.

FIG. 8 illustrates a method in accordance with the invention.

FIG. 9 illustrates a method in accordance with the invention.

FIG. 10 is a tabular representation of glass composition in accordancewith an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, exemplary embodiments disclosing specific details areset forth in order to provide a thorough understanding of the presentinvention. However, it will be apparent to one having ordinary skill inthe art having had the benefit of the present disclosure, that thepresent invention may be practiced in other embodiments that depart fromthe specific details disclosed herein. Moreover, descriptions ofwell-known devices, methods and materials may be omitted so as to notobscure the description of the present invention.

It is noted also that the refractive index change (Δn) as referencedherein is measured at 633 nm. In many occurances this is specificallynoted. However, if not specifically noted at a particular occurance, itis understood that the index change is measured at 633 nm.

Briefly, the present invention relates to meltable photosensitive glassmaterials. According to an exemplary embodiment of the presentinvention, a meltable photosensitive glass material has a molecularhydrogen content that is greater than 10¹⁷ hydrogen molecules/cm³. Whenexposed to light having a wavelength of less than 300 nm, the refractiveindex change in the exposed portions of the photosensitive glassmaterial is less than 10⁻⁴ (Δn<10⁻⁴) (measured at 633 nm). Usefully,gratings, as well as other structures, may be formed in the glassmaterial by selective exposure of the glass material to light of awavelength range at which the glass material is photosensitive. Theglass may be a germanosilicate glass. Germanosilicate glasses generallyhave a GeO₂ content of at least 2 weight %, and a SiO₂ content of atleast 20 weight %. In one illustrative embodiment of the presentinvention, the GeO₂ content of the germanosilicate glasses of thepresent invention is at least 6 weight %.

According to an exemplary embodiment, the present invention comprises abelow 300 nm UV light photosensitizable glass with 40-80% SiO₂, 2-15mole % GeO₂, 10-36 mole % B₂O₃, 1-6 mole % Al₂O₃ and 2-10 mole % R₂Owherein R is an alkali, with the glass photosensitive when loaded withhydrogen. Preferably, the glass comprises 42-73 mole % SiO₂, 2-15% mole% GeO₂, 25-36 mole % B₂O₃, 2-6 mole % Al₂O₃, and 2-6 mole % R₂O.Usefully, the glass comprises 42-67 mole % SiO₂, 2-15 mole % GeO₂, 25-36mole % B₂O₃, 2-6 mole % Al₂O₃, and 2-6 mole % R₂O. Preferably R₂O is atleast one alkali oxide chosen from the group of Na, Li, and K. In anembodiment R is Na. In another exemplary embodiment R is Li. In afurther exemplary embodiment R is K. Moreover, in embodiments of theinvention the R alkali content of the glass includes mixtures of Na, Li,and K. Preferably, the glass has an alkali/alumina mole ratio in therange of 1±0.5. Usefully, the glass is essentially free of non-bridgingoxygen ions and such are minimized and inhibited by the glasscomponents.

The invention further includes a molecular hydrogen loadablephotosensitive bulk glass comprised of an alkali boro-alumino-silicateglass with a melting temperature≦1650° C. Preferably the alkaliboro-alumino-silicate glass has a batch composition of ≦85 mole % SiO₂,≧10 mole % B₂O₃, ≧2 mole % GeO₂, and a combined alkali and aluminacontent <20 mole % Al₂O₃+R₂O. Preferably the batch composition is ≦80mole % SiO₂ and ≧20 mole % B₂O₃. More preferably the batch compositionhas ≦70 mole % SiO₂ and 25 mole B₂O₃. Preferably the glass has a batchcomposition with a combined alkali and alumina content <16 moleAl₂O₃+R₂O.

In another illustrative embodiment of the present invention, thephotosensitizable bulk glass is a germanosilicate glass havingapproximately 25 weight % to approximately 45 weight % SiO₂,approximately 3 weight % to approximately 22 weight % GeO₂,approximately 7 weight % to approximately 28 weight % B₂O₃,approximately 6 weight % to approximately 22 weight % Al₂O₃,approximately 6 weight % to approximately 25 weight % R₂O wherein R isan alkali, and approximately 3 weight % to approximately 11 weight % F,with the glass photosensitive when loaded with hydrogen. Illustratively,the glass comprises approximately 30 weight % to approximately 40 weight% SiO₂. Illustratively the glass has a GeO₂ content of approximately 7weight % to approximately 17 weight %. Illustratively the glass has aB₂O₃ content of approximately 10 weight % to approximately 22 weight %.Illustratively the glass has an Al₂O₃ content of approximately 10 weight% to approximately 19 weight %. Illustratively the glass has an R₂Ocontent of approximately 10 weight % to approximately 20 weight %.Illustratively, the glass has a F content of greater than about 5 weight%. Illustratively R₂O is at least one alkali chosen from the group ofNa, Li, and K. In an embodiment R is Na. In another exemplary embodimentR is Li. In a further exemplary embodiment R is K. Moreover, inembodiments of the invention the R alkali content of the glass includesmixtures of Na, Li, and K. Preferably, the glass has an alkali/aluminamole ratio (mole % R₂O/mole % Al₂O₃) in the range of approximately 0.5to approximately 1.5. Usefully, the glass is essentially free ofnon-bridging oxygen ions and such are minimized and inhibited by theglass components.

In accordance with an exemplary embodiment of the present invention, thephotosensitizable glass has a hydrogen content that is ≧10¹⁷H₂molecules/cm³. The hydrogen may be loaded in the glass by knowntechniques.

It is noted that according to an exemplary embodiment the glass has aloadable hydrogen content ≧10¹⁹H₂ molecules/cm³. Preferably, the glassis loadable and loaded with at least 2×10¹⁹H₂ molecules/cm³, andpreferably the hydrogen content is preferably at least 5×10¹⁹ hydrogenmolecules/cm³ according to another exemplary embodiment. Such hydrogenload levels are preferably achieved with a hydrogen loading temperatureno greater than 200° C. with the molecular hydrogen entering the glassas molecular hydrogen (H₂) and remaining as molecular hydrogen in theglass in that the hydrogen molecules contained in the glass do notdisassociate and react with the glass until irradiated.

The photosensitizable glass has a transition metal contaminant level ≦1ppm by weight for transition metal contaminants. The glass also has aheavy metal contaminant level ≦1 ppm by weight for heavy metalcontaminants. Preferably the glass has a Fe content ≦1 ppm by weight Fe,and more preferably <1 ppm by weight Fe. Preferably the glass has a Ticontent ≦1 ppm by weight Ti, and more preferably <1 ppm by weight Ti.The photosensitizable glass has a 300 nm absorption less than 30 dB/cm,usefully less than 20 dB/cm, and more preferably less than 15 dB/cm.Even more preferred the 250 nm absorption is 10 dB/cm and mostpreferably <5 dB/cm.

Preferably the photosensitizable glass is melted glass, and mostpreferably a non-sintered glass. The glass has a meltingtemperature≦1650° C., and preferably ≦1600° C. which provides forformation by melting a mixed batch of glass feedstock powders for form ahomogeneous glass melt which can be cooled into the glass. Preferablythe glass has a melting temperature≦1550° C., and more preferably ≦1500°C. Preferably the glass has a softening temperature≦700° C. Such glassforming temperatures allow for efficient and economic manufacturing ofthe glass and avoid the complications of sintering and sintered glasscompositions.

Preferably the glass has a below 300 nm wavelength induced modulatedrefractive index Δn>10⁻⁴ (measured at 633 nm) where the exposurewavelength is no greater than 300 nm and the glass has a molecularhydrogen content >10¹⁷H₂ molecules/cm³. According to an exemplaryembodiment, the inventive glass exhibits photosensitivity as aconsequence of exposure to light of no greater then 300 nm wavelengthinduced modulated refractive index Δn>10⁻⁴ when loaded with a molecularhydrogen content >10¹⁹ molecules/cm³. It is noted that the preferableexposure wavelength range to induce the above referenced wavelengthinduced modulated refractive index is approximately 248 nm toapproximately 265 nm. Preferably the glass has a modulated refractiveindex Δn>2×10⁻⁴ when hydrogen loaded. It is further noted that theexposure wavelength may be as low as 244 nm.

Preferably the photosensitive bulk glass is essentially free oftransition metals and with a 300 nm absorption less then 30 dB/cm.Preferably, the transition metal contaminant levels are below 1 ppm byweight, with the iron content <1 ppm by weight and more preferably <0.1ppm by weight. Preferably the titanium content is <1 ppm by weight, andmore preferably <0.1 ppm. Preferably the bulk glass has 300 nmabsorption <20 dB/cm, more preferably <15 dB/cm, more preferably <10dB/cm, and most preferred <5 dB/cm. It is noted that it is useful tominimize the impurity levels of iron, other transition metals and OH.Low concentrations of these impurities aids in reducing loss, which willimprove the uniformity of gratings formed in the glass materials inaccordance with an exemplary embodiment of the present invention.

The glass has a refractive index photosensitivity level modulatedΔn>10⁻⁵ (measured at 633 nm) with a loaded molecular hydrogen content≧10¹⁸H₂ molecules/cm³. Such a Δn can be achieved with a 248 nm KrFexcimer laser UV exposure of 90 minutes at 12 mJ/cm²/pulse. Preferablythe glass has a refractive index photosensitivity level Δn>10⁻⁴ with aloaded molecular hydrogen content ≧10¹⁹H₂ molecules/cm³. Preferably thebulk glass is loadable with molecular hydrogen to a molecular hydrogenloaded level of at least 10¹⁹H₂ molecules/cm³ with a hydrogen loadingtemperature≦200° C. Preferably the glass has a molecular hydrogencontent >10¹⁹H₂ molecules/cm³ and a below 300 nm wavelength inducedmodulated refractive index Δn>10⁻⁴(measured at 633 nm).

The bulk glass is a non-sintered glass, and preferably has a meltingtemperature≦1,600° C., and more preferred ≦1,550° C. Preferably theglass is a cooled fluid melt mixture formed from a fluid melt,preferably with the fluid melt formed by melting glass batch feedstockpowders. In a preferred embodiment the bulk glass is a homogeneous glassdevice preform body with a homogeneous composition with glass dopantsevenly spread throughout the glass body. Preferably the preform body hasa homogeneous index of refraction and is free of pre-radiated core andcladdings regions with a homogeneous distribution of glass componentelements.

The present invention also includes meltable germanosilicate glassesthat are photosensitive in the absence of hydrogen or at low levels ofhydrogen loading (for example, less than approximately 10¹⁷H₂molecules/cm², illustratively less than approximately 10¹⁴H₂molecules/cm³.) These germanosilicate glasses have a GeO₂ content of atleast 2 weight % (illustratively at least 6 weight %), and a SiO₂content of at least 20 weight %. Preferably, the meltablegermanosilicate glasses have melting temperatures of ≦1650° C., morepreferably ≦1550° C., and most preferably ≦1500° C. Illustratively, theglasses have a below 300 nm wavelength induced modulated refractiveindex Δn>10⁻⁴.

An exemplary embodiment of the present invention comprises a below 300nm UV light photosensitive glass with approximately 40 mole % toapproximately 80 mole % SiO₂, approximately 2 mole % to approximately 15mole % GeO₂, approximately 10 mole % to approximately 36 mole % B₂O₃,approximately 1 mole % to approximately 6 mole % Al₂O₃ and approximately2 mole % to approximately 10 mole % R₂O wherein R is an alkali, with theglass photosensitive in the absence of hydrogen and at low hydrogenloadings. Illustratively, the glass comprises approximately 42 mole % toapproximately 73 mole % SiO₂, approximately 2 mole % to approximately15% mole % GeO₂, approximately 25 mole % to approximately 36 mole %B₂O₃, approximately 2 mole % to approximately 6 mole % Al₂O₃, andapproximately 2 mole % to approximately 6 mole % R₂O. Usefully, theglass comprises approximately 42 mole % to approximately 67 mole % SiO₂,approximately 2 mole % to approximately 15 mole % GeO₂, approximately 25mole % to approximately 36 mole % B₂O₃, approximately 2 mole % toapproximately 6 mole % Al₂O₃, and approximately 2 mole % toapproximately 6 mole % R₂O. Illustratively, R is at least one alkalichosen from the group of Na, Li, and K. In one exemplary embodiment R isNa. In another exemplary embodiment R is Li. In a further exemplaryembodiment R is K. Moreover, in embodiments of the invention the Ralkali content of the glass includes mixtures of Na, Li, and K.Preferably, the glass has an alkali/alumina mole ratio in the range ofapproximately 0.5 to approximately 1.5. Usefully, the glass isessentially free of non-bridging oxygen ions and such are minimized andinhibited by the glass components.

In another embodiment of the present invention, the photosensitive bulkglass may be a germanosilicate glass having approximately 25 weight % toapproximately 45 weight % SiO₂, approximately 3 weight % toapproximately 22 weight % GeO₂, approximately 7 weight % toapproximately 28 weight % B₂O₃, approximately 6 weight % toapproximately 22 weight % Al₂O₃, approximately 6 weight % toapproximately 25 weight % R₂O wherein R is an alkali, and approximately3 weight % to approximately 11 weight % F, with the glass photosensitivein the absence of hydrogen and or at low hydrogen loadings.Illustratively, the glass comprises approximately 30 weight % toapproximately 40 weight % SiO₂. Illustratively the glass has a GeO₂content of approximately 7 weight % to approximately 17 weight %.Illustratively the glass has a B₂O₃ content of approximately 10 weight %to approximately 22 weight %. Illustratively the glass has an Al₂O₃content of approximately 10 weight % to approximately 19 weight %.Illustratively the glass has an R₂O content of approximately 10 weight %to approximately 20 weight %. Illustratively, the glass has a F contentof greater than about 5 weight %. Illustratively R₂O is at least onealkali chosen from the group of Na, Li, and K. In an embodiment R is Na.In another exemplary embodiment R is Li. In a further exemplaryembodiment R is K. Moreover, in embodiments of the invention the Ralkali content of the glass includes mixtures of Na, Li, and K.Preferably, the glass has an alkali/alumina mole ratio (mole % R₂O/mole% Al₂O₃) in the range of approximately 0.5 to approximately 1.5.Usefully, the glass is essentially free of non-bridging oxygen ions andsuch are minimized and inhibited by the glass components.

The photosensitive glass has a transition metal contaminant level ≦1 ppmby weight for transition metal contaminants. The glass also has a heavymetal contaminant level ≦1 ppm by weight for heavy metal contaminants.Preferably the glass has a Fe content ≦1 ppm by weight Fe, and morepreferably <1 ppm by weight Fe. Preferably the glass has a Ti content ≦1ppm by weight Ti, and more preferably <1 ppm by weight Ti. Thephotosensitive glass has a 300 nm absorption less than 30 dB/cm,usefully less than 20 dB/cm, and more preferably less than 15 dB/cm.Even more preferred the 250 nm absorption is 10 dB/cm and mostpreferably <5 dB/cm.

Preferably the photosensitive glass is melted glass, and most preferablya non-sintered glass. The glass has a melting temperature≦1650° C., andpreferably ≦1600° C. which provides for formation by melting a mixedbatch of glass feedstock powders for form a homogeneous glass melt whichcan be cooled into the glass. Preferably the glass has a meltingtemperature≦1550° C., and more preferably ≦1500° C. Preferably the glasshas a softening temperature≦700° C. Such glass forming temperaturesallow for efficient and economic manufacturing of the glass and avoidthe complications of sintering and sintered glass compositions. In apreferred embodiment the bulk glass is a homogeneous glass devicepreform body with a homogeneous composition with glass dopants evenlyspread throughout the glass body. Preferably the preform body has ahomogeneous index of refraction and is free of pre-radiated core andcladdings regions with a homogeneous distribution of glass componentelements.

Illustratively, the glass has a below 300 nm wavelength inducedmodulated refractive index Δn>10⁻⁴ (measured at 633 nm) where theexposure wavelength is no greater than 300 nm and the glass has no orlow hydrogen loading. According to an exemplary embodiment, theinventive glass exhibits photosensitivity as a consequence of exposureto light of no greater then 300 nm wavelength induced modulatedrefractive index Δn>10⁻⁴. It is noted that the preferable exposurewavelength range to induce the above referenced wavelength inducedmodulated refractive index is approximately 248 nm to approximately 265nm. Preferably the glass has a modulated refractive index Δn>2×10⁻⁴. Itis further noted that the exposure wavelength may be as low as 244 nm.The low or no hydrogen loaded photosensitive glasses may require ahigher level of irradiation to yield an index change equivalent to thatof the analogous hydrogen-loaded glass. For example, under a given UVfluence, a photosensitive glass may achieve an index change of 10⁻⁴after 8 minutes when hydrogen loaded, and after 64 minutes in theabsence of hydrogen.

The invention includes a method of making a refractive index pattern.Preferably the method comprises making a refractive index patterngrating. The method of making a pattern includes providing aphotosensitive bulk glass having a 300 nm absorption less than 30 dB/cm,preferably less than 20 dB/cm. The method includes providing a below 300nm radiation source and producing below 300 nm radiation. The methodincludes forming a pattern with the below 300 nm radiation and exposingthe photosensitive bulk glass to the pattern to form a modulatedrefractive index pattern in the bulk glass. Preferably the provided bulkglass has a <15 dB/cm absorption at 300 nm, more preferably <10 dB/cm,and most preferably <5 dB/cm. Forming the pattern preferably comprisesforming a pattern and exposing the bulk glass to the pattern for form amodulated refractive index grating in the bulk glass.

Providing the photosensitive bulk glass preferably includes providing analkali boro-alumino-silicate glass. The provided bulk glass bodypreferably is homogeneous in composition and refractive index and doesnot have separate core/cladding regions.

Providing the photosensitive bulk glass includes providing anon-sintered glass, with the glass being a melted glass. Preferably theglass is a melted glass with a melting temperature≦1,650° C. Morepreferably the melting temperature of the bulk glass ≦1,600° C., morepreferred ≦1,550° C., and most preferred ≦1,500° C. Providing thephotosensitive bulk glass includes providing an alkaliboro-alumino-silicate glass batch and melting the glass batch to form analkali boro-alumino-silicate glass melt. The method includes cooling theglass melt into the bulk glass. Preferably melting includes containingthe glass melt in a heated glassy fluid state and forming the glass meltinto a coolable body, such as delivering the glass melt through anorifice and to a cooling site.

In one embodiment of the present invention, the provided bulk glass maybe a molecular hydrogen loadable bulk glass. Preferably the methodincludes providing a melted bulk glass and loading the bulk glass withat least 10¹⁸H₂ molecules/cm³. Preferably loading the bulk glassincludes loading with at least 10¹⁹H₂ molecules/cm³, and more preferablyat least 5×10¹⁹H₂ molecules/cm³. Loading bulk glass is preformed with amolecular hydrogen loading temperature<200° C. Preferably the hydrogenloading temperature≦150° C., more preferably ≦100° C. Preferably ahydrogen load atmosphere of at least 20 atmospheres of hydrogen areused, and most preferably at least 100 atmospheres of H₂ is utilized todope the bulk glass. Such loading can be achieved in high temperaturevessels that contain the H₂ gas and the bulk glass. Preferably the bulkglass body has a glass body physical size with glass volume and surfacearea to provide efficient loading of hydrogen, preferably with the bulkglass body being a near net shape of the preform and optical device itis made into. The bulk glass is exposed to the H₂ gas pressurizedatmosphere for a H₂ loading time sufficient and effective such that thecenter of the bulk glass body has a molecular hydrogen concentrationthat is at least 90% of the ambient H₂ loading atmosphere. As theskilled artisan will appreciate, the provided bulk glass may also be aglass that is photosensitive in the absence of hydrogen loading or atlow hydrogen loadings.

Exposing the photosensitive bulk glass preferably includes exposing theglass to form a pattern by inducing a refractive index Δn≧10⁻⁵, and mostpreferably Δn≧10⁻⁴ (again measured at 633 nm).

The invention includes a method of making a molecular hydrogen loadablephotosensitive glass optical device preform. The method of making thepreform includes providing a germania silica glass batch with atransition metal contamination level ≦1 ppm by weight for heavy metals.The method includes melting the silica glass batch to form a homogeneousglass melt, cooling the glass melt into a UV transmitting bulk glasshaving a 300 nm absorption less than 20 dB/cm and forming the bulk glassinto an optical device preform. Forming the bulk into an optical devicepreform may include loading the bulk glass with molecular hydrogen to alevel of at least 10¹⁸H₂ molecules/cm³, and more preferably at least10¹⁹H₂ molecules/cm³.

Providing the germania silica glass batch includes providing an alkaliboro-alumino-silicate glass batch and melting the glass at a meltingtemperature≦1,650° C. Preferably melting comprises melting at ≦1,600°C., more preferably ≦1,550° C., and most preferably ≦1,500° C. Themethod of making preferably includes pouring the glass melt to form bulkglass bodies, and more preferably includes delivering the glass meltthrough a glass forming orifice. Making the bulk glass performspreferably includes forming a preform glass body with a smallest sizedimension that is greater than 5 μm.

The invention further includes a photosensitive glass optical refractiveindex pattern preform for use with UV light in the formation ofrefractive index patterns. The inventive preform is comprised of analkali boro-alumino-silicate glass with a 300 nm absorption less than 20dB/cm. The preform has a below 300 nm UV wavelength inducible modulatedrefractive index Δn≧10⁻⁵ with the bulk glass exhibiting photosensitivityas a consequence of exposure to light of 300 nm or less. The glass mayhave a molecular hydrogen level of at least 10¹⁸H₂ molecules/cm³ duringthe exposure. Preferably the refractive index pattern preform has a UVwavelength inducible modulated refractive index Δn level >10⁻⁴ with amolecular hydrogen level of at least 10¹⁹H₂ molecules/cm³. Preferablythe bulk glass preform has a 300 nm absorption less than 15 dB/cm, morepreferably less than 10 dB/cm, and most preferred less than 5 dB/cm.Preferably the alkali boro-alumino-silicate glass preform is anon-sintered glass body formed by a melting process is result in amelted glass.

EXAMPLES

The invention includes a large UV-induced refractive index change in amelted alkali-alumino-boro-germano-silicate composition that has beenloaded with molecular hydrogen. The UV exposures utilized include CW244-nm light and a pulsed KrF excimer laser at 248-nm. It is noted thata tuneable Nd/YAG laser which emits radiation at 268 nm, 270 nm, 280 nmand 290 nm could be used in place of the KrF excimer laser. A modulatedrefractive index of the order of 2-3×10⁻⁴ has been measured (at 633 nm)in the bulk glass.

It is believed that the ability to load with molecular hydrogen, and thephotoreaction, depends on the composition of the glass. The UVspectroscopy of the bulk glass before and after exposure, as well as themagnitude of the induced refractive index correlates well with thegrowth of the OH absorption as measured in the IR (OH stretchingvibrations).

As shown in FIG. 6, a Bragg grating was made in a bulk glass ample(Glass 5 g, Glass Composition Table) by exposing through a phase maskfrom the top face, with a measured transmission and reflectivity asshown.

In order to provide melted glasses, the invention utilizes variousconstituents to make the glass softer and lower the melting temperature.This includes using constituents like alkali, alumina and boron to lowerthe melting temperature and to decrease the viscosity. In a preferredembodiment the glass batch melting temperature is lowered by using asufficient amount of a fluoride of the glass components to lower themelting temperature. For example with Glass 4 b (See Glass CompositionTable of FIG. 10) aluminum fluoride is utilized with a F batchcomposition of about 3.3 wt. %. In a preferred embodiment the batchcomposition melting temperature is lowered with a batch compositionincorporating of fluorine at a batch wt. % of ≦10 weight % F,illustratively ≦6 weight % F. The lowering of the melting temperature isdone in such a way as not to move the fundamental absorption beyond248-nm (5-eV).

The fundamental absorption edge of pure silica, for example, isdetermined by the transition from the band consisting of the overlapping2 p oxygen orbitals (valence band) to the band made up from the sp³non-bonding orbitals of silicon (conduction band). The addition ofalkali introduces another set of levels associated with non-bridgingoxygen. When the concentration is high enough, a new band appears abovethat of the original valence band, thus moving the fundamentalabsorption edge to longer wavelengths. On the other hand, the additionof the network substitution ions such as boron, aluminum, and germaniumhas much less influence on the absorption edge.

Impurities such as transition metal ions or heavy metal ions that areinadvertently incorporated into glass, either from the batch materials,the containment crucible, the furnace or forming, must be kept to the <1ppm level. These ions, even in small amounts have a dramatic adverseeffect on the UV-absorption edge.

The invention includes making a SiO₂—GeO₂ bulk glass that can be meltedand formed in a conventional batch way be limiting the additionalconstituents sufficiently so as to maintain high transparency at 248-nm,and yet achieve melting at a reasonable temperature (1650° C.) and asoftening temperature of approximately of 600° C. (softening temperaturebelow 700° C. preferred).

Glasses were made from pure starting materials, in particular low ironcontent sand. They were melted in clean platinum crucibles at 1,550° C.for 16 hours. In the initial sampling procedure, the glass was pouredinto patties and annealed. Subsequently, the quality of the glass wasimproved in terms of (striae) defects and cords by using semi-continuousmelting where the glass is not poured from a melt crucible which is thesource of much of the striae, but delivered through an orifice.

The hydrogen loading was done in a Parr™ pressurized reactor using 150°C. loading temperature at 100 atm pressure. The IR spectroscopy was donewith a Nicolet™ FTIR spectrometer to determine the molecular hydrogen(H₂) content.

The effect on the absorption spectrum with change in GeO₂ content foralkali-alumino-boro-silicate glass family of R₂O (3-4 mole %), Al₂O₃(3-4 mole %), B₂O₃ (25-35 mole %), GeO₂ (2.5-15 mole %), and SiO₂(66.5-42 mole %) is shown in FIG. 1. In all cases we were able tomaintain high transmittance at below 300 nm wavelength of 248-nm whichwas to be the UV exposure excitation wavelength.

We exposed samples from each system to pure hydrogen ambient at 100 atmat 150° C. in order to impregnate the samples with molecular hydrogen.The higher temperature was used to speed up the diffusion process forsamples that were a few millimeters thick and yet not allow the thermalreaction to occur. We used IR spectroscopy to determine the molecularhydrogen content. Photosensitivity was achieved with the inventiveglasses that had a loaded molecular hydrogen content. Hydrogen loadlevels up to 5×10¹⁹H₂ molecules/cm³ were obtained with the inventiveglass.

In order to measure the UV-induced photosensitivity, we hydrogen loaded0.5-mm thick bulk glass samples. We then exposed them through a chromeabsorption mask with a 10 μm grating pitch. The UV exposure source was aKrF excimer laser operating at 248-nm. The peak fluence was from 20-60mJ/cm²/pulse at 50 Hz for periods of time running from 5-120 min.

After the UV exposure, the sample was illuminated by a spatiallyfiltered He-Ne laser and the diffraction efficiency of the induced phasegrating was measured from the ratio of the intensity of the 1^(st) to0^(th) order. As long as the diffraction efficiency is relatively weakone can use the following simple formula for efficiency. $\begin{matrix}{{Eff} = \left( \frac{\pi \quad \Delta \quad {nL}}{\lambda} \right)^{2}} & (1)\end{matrix}$

Here, Δn is the modulated refractive index change (n=n₀+Δn cos(2πz/Λ)),L is the grating index and Λ is the period of the index pattern.

The range of measured values of the induced refractive index after afixed 248-nm UV exposure was from 1×10⁻⁴ to 3×10⁻⁴ for the inventivealkali-alumino-boro-silicate glasses. The induced modulated refractiveindex as a function of the exposure time at fixed fluence is shown inFIG. 2a. FIG. 2b shows the measured induced index as a function offluence at fixed time. The later is well represented by using the squareof the fluence.

A set of glasses with fixed germania content was loaded with H₂ and UVexposed at 248-nm. The Glass Composition Tables gives the compostion,relative amounts of H₂ incorporated and the 248-nm excimer laser inducedrefractive index change for the set.

The thermal stability of the induced refractive index change wasinvestigated by heating a sample having a grating and re-measuring thegrating efficiency with time at temperature. The change with time afterheating to 400 degrees is shown in FIG. 3.

We have produced UV-induced refractive index changes in meltednon-sintered bulk glasses, similar in magnitude to that seen in vapordeposition sintered flame or plasma prepared glasses, when a highconcentration of molecular hydrogen can be obtained. This followsdirectly from our observation that preferred melted glasses where we seemolecular H₂ (>10¹⁸/cm³, as estimated by IR spectroscopy) we preferablyobtain a measureable UV-induced refractive index change.

The mechanism for the molecular-mediated UV-induced photosensitivityseems to be consistent with that proposed for the hydrogen mediatedeffect found in the SiO₂—GeO₂ material prepared by methods other thanmelting such as vapor deposition and sintering. FIG. 4 shows therelationship of the increase in hydroxyl concentration (measured fromthe OH stretching vibration; see inset) with the induced refractiveindex also as shown in FIG. 5. This is also a large change in UVabsorption after exposure.

We have observed stress that derives from the UV-exposure. This suggeststhat a volume change is occurring. From the sign of the birefringence wedetermine that the volume change corresponds to a densification. Therefractive index contribution computed from this effect is smallcompared to the overall measured Δn.

As shown in FIG. 6, a bulk glass sample of Glass 5g of the GlassComposition Table (glass block of 5×5×3 mm³) was exposed through thewide face using a 244-nm CW laser (0.35W for preferred UV exposure timeof 30-60 minutes) utilizing a phase mask with a period such as tosatisfy the Bragg condition at 1,550-nm to produce a refractive indexpattern. The grating length was 2.5-nm. The reflectivity andtransmission of the grating is shown in FIG. 6. The inset shows thegeometry of the exposure and the reflectivity and transmissionmeasurement. From the grating transmission measurement (1.5-2 dbdecrease corresponding to 30-40% reflectivity in the 2.5-mm longgrating), a modulated refractive index change of 0.12-0.14×10⁻³ iscalculated at 1,550-nm. FIG. 7 shows the refractive index patterngrating formed in the bulk glass preform glass block. FIG. 7a is across-section showing the refractive index pattern grating in the bulkglass preform glass block.

While the UV exposure experiments described in this Example have usedhydrogen loading to increase the photosensitivity of the glasses, it isnoted that the glasses of this Example are also photosensitive in theabsence of hydrogen and at low hydrogen loading levels. Glasses withno/low hydrogen loading may require a longer exposure time or higherexposure fluence to achieve a given index change.

Glass 4 b of the Composition Table is preferred composition of theinvention. The weight percent batch composition was 35.8 wt. % SiO₂,21.5 wt. % GeO₂, 4.48 wt. %, Al₂O₃, 3.38 wt. % F, 1.31 wt. Li₂O, and33.5 wt. % B₂O₃. The batch material powders were ball milled to providea homogeneous batch mix. For SiO₂, high purity silica sand powder, achas IOTA-6 brand SiO₂ from the Unimin Corporation, Spruce Pine, N.C.28777, was utilized with the high purity silica sand having a Feimpurity level of less than 0.1 ppm. For GeO₂, high purity germaniumdioxide powder such as Chemical Grade No. 1-29/99.999% purity GeO₂ fromthe Electro-Optic Materials Dept., Eagle-Picher Technologies, LLC,Quapaw, Okla. 74363, was utilized with a purity having 0.1% maximumchloride content, ≦1 ppm Fe, ≦1 ppm Mg, ≦0.5 ppm Ni, and no detectablePb (1 ppm detect limit), and no Zn (10 ppm detect limit). For aluminum,high purity aluminum oxide powder, such as Gamma brand aluminum oxide99.999% from Alfa Aesar, A Johnson Mathey Company, Ward Hill, Mass.01835, was utilized with the 99.999% purity. For aluminum, high purityaluminum fluoride was also used, such as Alufluor brand aluminumfluoride from LidoChem, Hazlet, N.J. 07730. For lithium, lithiumcarbonate was used, such as the Tech Grade 99+% purity Li₂CO₃ brand fromFMC Corporation, Lithium Div., Gastonia, N.C. 28054, with a 99%+ purity,with Fe₂O₃ wt. ≦0.004 and a Cl wt. % ≦0.01. Also for lithium, lithiumnitrate crystal was used, such as available from VWR Scientific,Rochester, N.Y. 14603. For boron, boric oxide was used, such as Hipurity brand Anhydrous Boric Acid from Stetson Chemicals, Inc., 391Exchange St., Buffalo, N.Y. 14204. The batch powder mixture after ballmilling was loaded into a large vertically oriented platinum linescylindrical furnace 100 as shown in FIG. 8. A total batch mass of 25 kgwas used with furnace 100 in a semi-continuous run.

Batch powder mixture 101 was melted at 1,550° C. Furnace 100 includes astirrer 102 for stirring glass melt 104 to provide a homogeneous glassmelt. Furnace 100 includes a down corner 106 and a down corner orifice108 for delivering a bulk glass body 110. Bulk glass bodies 110 weremade with a general dimension of 105×4×4 inches (3.81×10.16×10.16 cm)and annealed at 414° C. The annealed bulk glass bodies 110 were cut,finished, and polished to provide small bulk glass bodies 120 having arectangular block shape. Bulk glass bodies 120 had a dimension of 5×5×3mm³. As shown in FIG. 9, bulk glass bodies 120 were loaded withmolecular hydrogen (H₂) in a hydrogen pressure vessel 200 using hydrogenatmosphere 210 of about 100 atmospheres to provide H₂ loaded bulk glassbody preforms 120.

Other exemplary glasses of the present invention have higher alumina andfluorine contents than the glasses of the Composition Table. Forexample, glass 18 has a composition including 34.7 wt % SiO₂, 11.2 wt %GeO₂, 16.4 wt % Al₂O₃, 1.66 wt % Na₂O, 12.9 wt % K₂O, 15.9 wt % B₂O₃,and 7.23 wt % F. Glass 19 has a composition including 34.51 wt % SiO₂,13.6 wt % Al₂O₃, 1.32 wt % Na₂O, 10.8 wt % K₂O, 15.81 wt % B₂O₃, and7.23 wt % F. The glasses were melted using methods analogous to thosedescribed above with reference to Glass Composition Table 1. Theseglasses have hydrogen-loaded and low/no hydrogen-loadedphotosensitivities roughly equivalent to those described above.

The invention having been described in detail in connection through adiscussion of exemplary embodiments, it is clear that modifications ofthe invention will be apparent to one having ordinary skill in the arthaving had the benefit of the present disclosure. Such modifications andvariations are included in the scope of the appended claims.

We claim:
 1. A meltable photosensitive glass having a molecular hydrogencontent of ≧10¹⁷H₂ molecules/cm³ and a melting temperature<1.650° C. 2.A glass as recited in claim 1, wherein the glass is a germanosilicateglass.
 3. A glass as recited in claim 1 wherein the glass comprises analkali boro-alumino-silicate having a batch composition comprising ≦85mole % SiO₂, ≧10 mole % B₂O₃, ≧2 mole % GeO₂, and a combined alkalimetal an alumina content <20 mole % Al₂O3+R₂O, said glass having amolecular hydrogen loadable level of at least 10¹⁸H₂ molecules/cm³.
 4. Aglass as recited in claim 3, having a batch composition with ≦70 mole %SiO₂ and ≧25 mole % B₂O₃.
 5. A glass as recited in claim 1, wherein theglass is photosensitizable to light having a wavelength of less than 300nm, and wherein said glass comprises 40-80 mole % SiO₂, 2-15 mole %GeO₂, 10-36 mole % B₂O₃, 1-6 mole % Al₂O₃, and 2-10 mole % R₂O wherein Ris an alkali metal.
 6. A glass as recited in claim 5, wherein said glasscomprising 42-73 mole % SiO₂, 2-15 mole % GeO₂, 20-36 mole % B₂O₃, 2-6mole % Al₂O₃, and 2-8 mole % R₂O.
 7. A glass as recited in claim 5,wherein said glass comprising 4-67 mole % SiO₂, 2-15 mole % GeO₂, 25-36mole % B₂O₃, 2-6 mole % A1203, and 2-6 mole % R₂O.
 8. A glass as recitedin claim 5, wherein R is at least one alkali metal chosen from a groupconsisting of Na, Li, and K.
 9. A glass as recited in claim 5, having atransition metal contaminant level of ≦1 ppm by weight for transitionmetal contaminants.
 10. A glass as recited in claim 5, having a heavymetal contaminant level ≦1 ppm by weight for heavy metal.
 11. A glass asrecited in claim 5, having a Fe content <1 ppm by weight Fe.
 12. A glassas recited in claim 5, having a Ti content <1 ppm by weight Ti.
 13. Aglass as recited in claim 5, wherein said glass as a alkalimetal/alumina ratio in the range of 1±0.5.
 14. A glass as recited inclaim 5, having an induced modulated refractive index Δn>10⁻⁴ byradiation having a wavelength below 300 nm.
 15. A glass as recited inclaim 1, wherein said molecular hydrogen content is loaded in said glassand said content is ≧10¹⁸H₂ molecules/cm³.
 16. A glass as recited inclaim 1, wherein said molecular hydrogen content is loaded in said glassand said content is ≧10¹⁹H₂ molecules/cm³.
 17. A glass as recited inclaim 1, having an absorption less than 20 dB/cm at 300 nm.
 18. A glassas recited in claim 1, said glass having a modulated refractive indexΔn≧2×10⁻⁴.
 19. A glass as recited in claim 1, having a meltingtemperature≦1,6000° C.
 20. A glass as recited in claim 1, having amelting temperature≦1,5500° C.
 21. A glass as recited in claim 1, havinga melting temperature≦1,5000° C.
 22. A glass as recited in claim 1,having a softening temperature≦700° C.
 23. A glass as recited in claim1, having an induced modulated refractive index Δn>10⁻⁴ by radiationhaving a wavelength below 300 nm when said molecular hydrogen content is>10¹⁹H₂ molecules/cm³.
 24. A glass as recited in claim 2, wherein saidglass having an increased OH content when loaded with molecular hydrogenand exposed to UV radiation.
 25. A glass as recited in claim 2, whereinsaid glass having an OH range of about 100 to 1,000 OH ppm weight.
 26. Aglass as recited in claim 1, wherein said glass comprises approximately25 weight percent to approximately 45 weight % SiO₂, approximately 3weight percent to approximately 22 weight % GeO₂, approximately 7 weight% to approximately 28 weight % B₂O₃, approximately 6 weight % toapproximately 22 weight % Al₂O₃, approximately 6 weight % toapproximately 25 weight % R₂O wherein R is an alkali metal andapproximately 3 weight % to approximately 11 weight F.
 27. A glass asrecited in claim 26, wherein said glass comprises approximately 30weight % to approximately 40 weight % SiO₂.
 28. A glass as recited inclaim 26 wherein said glass has a GeO₂ content of approximately 7 weight% to approximately 17 weight %.
 29. A glass as recited in claim 26wherein said glass has a B₂O₃ content of approximately 10 weight % toapproximately 22 weight %.
 30. A glass as recited in claim 26 whereinsaid glass has an Al₂O₃ content of approximately 10 weight % toapproximately 19 weight %.
 31. A glass as recited in claim 26 whereinsaid glass has an R₂O content of approximately 10 weight % toapproximately 20 weight %.
 32. A glass as recited in claim 26 whereinsaid glass has a F content of approximately 5 weight % to approximately11 weight %.
 33. A glass as recited in claim 26, wherein said glass hasan alkali metal/alumina ratio in the range of approximately 0.5 toapproximately 1.5.
 34. A glass as recited in claim 1 having a fluorinecontent of ≦10 wt %.
 35. A glass as recited in claim 1 having a fluorinecontent of ≦6 wt %.
 36. A meltable photosensitive germanosilicate glassmaterial having a hydrogen content less than approximately 10¹⁷H₂molecules/cm³.
 37. A glass as recited in claim 36, wherein the glasscomprises an alkali boro-alumino-silicate glass having a meltingtemperature≦1650° C., said alkali boro-alumino-silicate glass having abatch composition comprising ≦85 mole % SiO₂, ≧10 mole % B₂O₃, ≧2 mole %GeO₂, and a combined alkali metal and alumina content <20 mole %Al₂O₃+R₂O.
 38. A glass as recited in claim 37, having a batchcomposition with ≦70 mole % SiO₂ and ≧25 mole % B2O₃.
 39. A glass asrecited in claim 36, wherein the glass is photosensitizable to lighthaving a wavelength of less than 300 nm, and wherein said glasscomprises approximately 40 mole % to approximately 80 mole % SiO₂,approximately 2 mole % to approximately 15 mole % GeO₂, approximately 10mole % to approximately 36 mole % B₂O₃, approximately 1 mole % toapproximately 6 mole % Al₂O₃, and approximately 2 mole % toapproximately 10 mole % R₂O wherein R is an alkali metal.
 40. A glass asrecited in claim 39, wherein said glass comprises approximately 42 mole% to approximately 73 mole % SiO₂, approximately 2 mole % toapproximately 15 mole % GeO₂, approximately 20 mole % to approximately36 mole % B2O₃, approximately 2 mole % to approximately 6 mole % Al₂O₃,and approximately 2 mole % to approximately 8 mole % R₂O.
 41. A glass asrecited in claim 39, wherein said glass comprises approximately 42 mole% to approximately 67 mole % SiO₂, approximately 2 mole % toapproximately 15 mole % GeO₂, approximately 25 mole % to approximately36 mole % B₂O₃, approximately 2 mole % to approximately 6 mole % Al₂O₃,and approximately 2 mole % to approximately 6 mole % R₂O.
 42. A glass asrecited in claim 39, wherein R is at least one alkali metal chosen froma group consisting essentially of Na, Li, and K.
 43. A glass as recitedin claim 39, wherein said glass has an alkali metal/alumina ratio in therange of approximately 0.5 to approximately 1.5.
 44. A glass as recitedin claim 39, having an induced modulated refractive index Δn10⁻⁴ byradiation having a wavelength below 300 nm.
 45. A glass as recited inclaim 36 wherein the glass has a hydrogen content of less thanapproximately 10¹⁴ hydrogen molecules/cm³.
 46. A glass as recited inclaim 36, having an absorption less than 20 dB/cm at 300 nm.
 47. A glassas recited in claim 36, said glass having a modulate refractive indexΔn≧2×10⁻⁴.
 48. A glass as recited in claim 36, having a meltingtemperature≦1,650° C.
 49. A glass as recited in claim 36, having amelting temperature≦1,600° C.
 50. A glass as recited in claim 36, havinga melting temperature≦1,550° C.
 51. A glass as recited in claim 36,having a melting temperature≦1,500° C.
 52. A glass as recited in claim36, having a softening temperature<700° C.
 53. A glass as recited inclaim 36, wherein said glass comprises approximately 25 weight % toapproximately 45 weight % SiO₂, approximately 3 weight % toapproximately 22 weight % GeO₂, approximately 7 weight % toapproximately 28 weight % B₂O₃, approximately 6 weight % toapproximately 22 weight % Al₂O₃, approximately 6 weight % toapproximately 25 weight % R₂O wherein R is an alkali metal, andapproximately 3 weight % to approximately 11 weight % F.
 54. A glass asrecited in claim 53, wherein said glass comprises approximately 30weight % to approximately 40 weight % SiO₂.
 55. A glass as recited inclaim 53 wherein said glass has a GeO₂ content of approximately 7 weight% to approximately 17 weight %.
 56. A glass as recited in claim 53wherein said glass has a B₂O₃ content of approximately 10 weight % toapproximately 22 weight %.
 57. A glass as recited in claim 53 whereinsaid glass has an Al₂O₃ content of approximately 10 weight % toapproximately 19 weight %.
 58. A glass as recited in claim 53 whereinsaid glass has an R₂O content of approximately 10 weight % toapproximately 20 weight %.
 59. A glass as recited in claim 53 whereinsaid glass has a F content of approximately 5 weight % to approximately11 weight %.
 60. A glass as recited in claim 53, wherein said glass hasan alkali metal/alumina ratio in the range of approximately 0.5 toapproximately 1.5.
 61. A photosensitive glass optical refractive indexpattern preform for use with UV light in the formation of refractiveindex patterns, said preform comprised of an alkaliboro-aluminio-silicate glass with an absorption less than 20 dB/cm at300 nm, said refractive index pattern preform having a UV wavelengthinducable modulated refractive index Δn>10⁻⁵ with a molecular hydrogenlevel of at least 10¹⁸H₂ molecules/cm³.
 62. A refractive index patternpreform as recited in claim 61, having a UV wavelength inducablemodulated refractive index Δn>10⁻⁴ with a molecular hydrogen level of atleast 10¹⁹H₂ molecules/cm³.
 63. A refractive index pattern preform asrecited in claim 61, wherein said alkali boro-alumino-silicate glass isa melted glass.